1. Field of the Invention
The present invention relates to a developing device, and an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a copier, a printer, a facsimile machine, or a printer, an electrostatic latent image formed on a photosensitive member as an image carrier is visualized by a developing device to obtain a visible image, and the visible image is transferred onto a sheet and output.
Some image forming apparatuses have such a configuration that one photosensitive member is provided for only a single color, and others have such a configuration that a plurality of photosensitive members are provided to form an image of a plurality of colors. The latter configuration is used to form a multicolor image including a full-color image.
Meanwhile, developers for use in development include a one-component developer containing only a magnetic or non-magnetic toner and a two-component developer with a toner and a carrier being mixed therein.
The two-component developer includes a toner and a carrier that carries the toner. The toner is charged through frictional electrification caused by stirring and mixing to be in a state in which the toner can be electrostatically attached to the electrostatic latent image on the photosensitive member.
Japanese Patent Application Laid-Open No. 2003-270933, for example, discloses a developing device that supplies a two-component developer. The conventional developing device includes rotatable screw augers as first and second developer supplying units. The first and second developer supplying units are arranged in parallel from a position close to a developing roller disposed near a photosensitive member as an electrostatic-latent-image carrier in a developer container that contains the developer.
In the conventional developing device, one of the first and second screw augers that is near the developing roller supplies the developer to the developing roller as a main function, whilst the other one that is located away from the developing roller stirs new toner and carrier supplied to the storage unit for mixing as a main function.
Therefore, the first developer supplying unit is required to ensure a sufficient amount of drawn-up developer for the developing roller. The second developer supplying unit is required to keep an appropriate mixing ratio between the toner and the carrier to be stirred so as not to deteriorate a charging characteristic of the toner.
However, if the fluidity of the developer is decreased with time or the density of the toner is increased (abnormality in the supply amount occurs), an exchange of the developer between the first and second screw augers may become uneven (the balance of the supply amount of the developer between the screw augers is disturbed), which causes an abnormal image. That is, if the developer is decreased at the screw auger located near the developing roller, the amount of the developer to be drawn up is decreased, resulting in deterioration in image density and unevenness in development due to nonuniformity of the amount of the developer carried on the developing roller (stripes according to a screw pitch of the screw auger and unevenness in density).
Conversely, if the developer is increased at the screw auger located away from the developing roller, the fluidity may be decreased, which causes unevenness in density and an increase in density deviation.
It has been generally known that toner supply control is performed by forming a test pattern or the like through development and, based on the density of the test pattern, determining the density of the developer. However, management of the amount of developer and its deviation state in the developing device is not performed, and it is often the case that only the management control over the developer with the test pattern is performed, and management of the balance of the actual supply amount has not yet been performed. Moreover, even if the balance is disturbed, there is no solution to this problem under the present circumstances.
Many models of electrophotography devices in recent years have adopted a two-component development scheme using a powder toner and a carrier. Reasons for this include high durability and high responsivity to speedup, and that is why this scheme has been widely spread.
However, unlike the one-component development scheme, this scheme requires a mechanism for evenly mixing the toner and the carrier. Therefore, various contrivances are provided to the developing device of the two-component development scheme.
For example, a stirring member is provided to efficiently mix the toner and carrier, or a toner supply position is changed or a conveyor route is increased so as to increase the time of mixing the developer. More specifically, for example, a two-axis conveying scheme using two conveyor screws has been widely known and spread.
FIG. 13 is a schematic diagram of a general developing device 2 of a two-component developing system. The developing device is explained as having a typical configuration including two screws and one developing roller. However, the developing device is not meant to be restricted to such a configuration.
The developing device 2 includes a developing roller 3 and two developer conveyor screws 14 and 15. Of these developer conveyor screws 14 and 15, the one closer to the developing roller 3 is called a first developer conveyor screw 14 and the other closer to a toner supply opening (not shown) is called a second developer conveyor screw 15.
On the developer conveyor screws 14 and 15, a two-component developer containing a toner and a carrier is input in an evenly mixed state, and is circulated as being stirred between the two developer conveyor screws 14 and 15. This is a contrivance to efficiently stir the toner input from the toner supply opening provided at an end of the second developer conveyor screw 15.
In a one-axis conveyance scheme not depicted, the developer is drawn up by the developing roller to form an ear of the developer by a magnet incorporated in the developing roller. The ear strokes the surface of the photosensitive member opposing thereto with a predetermined distance being kept, thereby developing the toner onto the photosensitive member through an applied developing bias.
In the two-axis conveyance scheme explained above, ideally, the first developer conveyor screw 14 has a sufficient amount of developer to be sufficiently drawn up by the developing roller, and the second developer conveyor screw 15 has a sufficient amount required for stirring the toner supplied for achieving the original purpose and the developer.
For example, Japanese Patent Application Laid-Open Nos. 2003-270933 and 2005-227316 have proposed technologies related to the two-axis conveyor scheme for developer. Those technologies aim at ideally supplying to the first developer conveyor screw on a developer sleeve side a sufficient amount of developer to be drawn up by the developing roller and supplying to the second developer conveyor screw a sufficient amount of developer required to stir the toner and the developer.
Japanese Patent Application Laid-Open No. 2003-270933 describes that, in order to reduce stress onto the toner, a developer conveyor screw and a developing roller can be drive-controlled at arbitrary timing. In the conventional technology, rotation is stopped in order to reduce a developer stirring time as much as possible other than the developing operation. Japanese Patent Application Laid-Open No. 2005-227316 describes that, in order to start up the developer in a short time, a developer conveyor screw is driven earlier than usual at the time of power-on or recovery from sleep.
However, in the conventional technology explained in connection with FIG. 13, because of a decrease in fluidity of the developer with time, an increase in the amount of developer due to an increase in density of the toner, and the like, an appropriate balance of the amount of developer between the first and second developer conveyor screws 14 and 15 may be often decreased. With such a state, there is a problem of occurrence of an inconvenience on the image.
When the developer on the first developer conveyor screw 14 side is decreased, the amount of drawing up by the developing roller 3 is decreased, thereby causing a decrease in image density and a screw-pitched abnormal image.
Conversely, when the developer on the first developer conveyor screw 14 side is increased, fluidity is decreased, thereby causing unevenness in density on the right and left sides of the image and an increase in image density deviation in a page. Moreover, since the amount of developer and the state of deviation cannot be known from outside, only the abnormal image has to be used for determination. Therefore, there is no solution even when any problems explained above occur.